The present application claims priority based on Japanese Patent Application No. 2001-268909 filed on Sep. 5, 2001, the contents of which are herein incorporated by reference.
1. Field of the Invention
The present invention relates generally to a choke coil having a coil pair having winding ratio of 1:1. More particularly, the invention relates to a choke coil to be effectively used in a band-pass filter used as inserted in a transmission line balanced with respect to the ground, such as a telephone circuit and so forth.
2. Description of the Related Art
For example, in ADSL (Asymmetric Digital Subscriber Line) in which superimposed a high speed digital signal on a telephone circuit, a branching filter called as splitter is employed for separating a telephone signal and a digital signal. The branching filter is constructed with an LPF (low-pass filter) selectively passing a frequency band of the telephone signal and an HPF (high-pass filter) selectively passing a frequency band of the digital signal.
On the other hand, the telephone circuit, in which the branching filter is used, is typically a transmission line balanced with respect to the ground. Upon forming a BPF (band-pass filter), such as LPF, HPF and the like, it is essential to certainly achieve unbalance attenuation amount with respect to the ground in order to restrict adverse effect of potential fluctuation in a grounding layer and loop noise with the ground.
As shown by winding structure and equivalent circuits of a choke coil in FIGS. 4A, 4B, 4C and 4D, 4E and 4F, a choke coil is constituted by winding a coil pair L1 and L2 in one coil with a winding ratio of 1:1. Both coils L1 and L2 are wound on a common core (magnetic core) in a single coil bobbin. Various winding structures may be employed as shown in FIGS. 4A to 4F.
The coil shown in FIG. 4A includes a first coil L1 and a second coil L2 forming the coil pair having winding ratio of 1:1. The first and second coils L1 and L2 are wound inside and outside separated by an intermediate insulation layer 2. The first coil L1 is wound predetermined turns n1 inside of the intermediate insulation layer 2. On the other hand, the second coil L2 is wound predetermined turns n2 (n1=n2) outside of the intermediate insulation layer 2. By this, the choke coil having the coil pair L1 and L2 having winding ratio of n1:n2=1:1, is formed. It should be noted that outermost side of the coil is covered with an insulation layer 29.
The coil shown in FIG. 4B includes the first and second coils L1 and L2 having predetermined turns n1 and n2. Respective first and second coils L1 and L2 are dividedly wound the predetermined turns n1 and n2 into two coil segments respectively having number of turns half of the predetermined turns n1 and n2. From inside to outside, a first coil segment L11 of the first coil L1, a first coil segment L21 of the second coil L2, a second coil segment L12 of the first coil L1 and a second coil segment L22 of the second coil L2 are sequentially wound in sequentially stacked manner. Between respectively adjacent coil segments L11, L21, L12 and L22, intermediate insulation layers 2 are interposed for insulation therebetween. Winding numbers n11, n21, n12 and n22 of the coil segments L11, L21, L12 and L22 are the same (n11=n21=n12=n22) as each other. By this, the coil pair L1 and L2 having the winding ratio 1:1 is formed.
The coil shown in FIG. 4C also includes the first and second coils L1 and L2, wherein the first coil L1 is separated into the first coil segment L11 and the second coil segment L12 which are arranged on innermost side and outermost side. Between the first and second coil segments L11 and L12 of the first coil L1, the first and second coil segments L21 and L22 of the second coil L2 are wound simultaneously.
The coil shown in FIG. 4D has a partitioning wall portion 11 dividing a spool portion of the coil bobbin 1 into two sections. In respective sections, the first and second coils L1 and L2 are separately wound by the predetermined turns n1 and n2 (n1=n2).
The coil shown in FIG. 4E is also provided with the partitioning wall portion 11 dividing the spool of the coil bobbin 1 into two sections. In one of the divided spool sections, the first coil segment L11 of the first coil L1 and the first coil segment L21 of the second coil L2 are wound in stacking manner with insulation by the intermediate insulation layer 2. In the other divided spool section, the second coil segment L12 of the first coil L1 and the second coil segment L22 of the second coil L2 are wound in stacking manner with insulation by the intermediate insulation layer 2.
In the coil shown in FIG. 4F, the first coil L1 and the second coil L2 are simultaneously wound by respectively predetermined turns n1 and n2 (n1=n2).
As set forth above, there have been proposed various manners of winding in the common mode choke coil.
In the conventional choke coil set forth above, the inventors have found the following problems to be encountered.
Namely, in case of the coils shown in FIGS. 4A and 4B, while number of turns n1 and n2 of the first and second coils L1 and L2 are the same with each other to have 1:1 of winding ratio, difference of wire wound resistances due to difference of winding diameters of the first and second coils L1 and L2 is significant to cause difficulty in obtaining sufficient unbalance attenuation amount with respect to the ground.
In case of the coil shown in FIG. 4C, the first and second coils L1 and L2 are divided into two segments, respectively. The first and second coil segments L21 and L22 of the second coil L2 are simultaneously wound. Therefore, winding operation is complicated and difficult to increase process steps.
In case of the coils shown in FIGS. 4D and 4E, the bobbin provided with the partitioning wall portion 11 dividing the spool into two sections becomes necessary. Furthermore, in comparison with the divided winding structure (so-called sandwich structure) of coaxially stacking manner shown in FIG. 4A or 4B, a leakage inductance between the first and second coils L1 and L2 becomes large. If the leakage inductance becomes large, when the band-pass filter, such as LPF or the like is formed, good transmission characteristics cannot be obtained in a pass frequency band (pass band).
In case of the coil shown in FIG. 4F, line capacity between the first and second coils L1 and L2 becomes large. When the line capacity becomes large, if the band-pass filter is formed, good attenuation characteristics cannot be obtained at a blocking frequency band (out of pass-band).
As set forth above, the aforementioned conventional choke coils cause problems in unbalance attenuation amount with respect to the ground, workability in winding, leakage inductance, line capacity and so forth. Conventionally, it is quite difficult to clear up all of the problems set forth above.
The present invention has been worked out in view of the above and other problems. Therefore, it is an object of the present invention to provide a choke coil which has small difference of wire wound resistances between coils to obtain sufficient unbalance attenuation amount, has superior in workability in winding and reduce leakage inductance and line capacity.
According to one aspect of the present invention, a choke coil including a coil pair having a winding ratio of 1:1, comprises:
first and second coils forming the coil pair, respective of the first and second coils being divided into first and second segments wound on a single spool disposing intermediate insulations layers between respectively adjacent coil segments, the first and second coil segments being stackingly wound in sequential order of the first coil segment of the first coil, the first coil segment of the second coil, the second coil segment of the first coil and the second coil segment of the second coil from inside to outside,
the first coil segment of the first coil and the second coil segment of the second coil being wound in a first winding number and the second coil segment of the second coil and the first coil segment of the second coil being wound in a second winding number,
the second winding number being greater than the first winding number for reducing difference of winding resistance between the first and second coils less than or equal to 4%.
A thickness of the intermediate insulation layer disposed between the first coil segment of the second coil and the second coil segment of the first coil may be adjusted for adjusting the difference of winding resistance between the first and second coils less than or equal to 4%.
Thicknesses of the insulation layers may be set to be different from each other so as to establish the difference of winding resistance between the first and second coils less than or equal to 4%. For example, a thickness of an insulation layer disposed between the first coil segment of the second coil and the second coil segment of the first coil may be differentiated from thicknesses of insulation layers respectively disposed between the first coil segment of the first coil and the first coil segment of the second coil and between the second coil segment of the first coil and the second coil segment of the second coil. Also, the intermediate insulation layer may be formed by winding an insulation tape for adjusting the difference of winding resistance between the first and second coils less than or equal to 4% by thickness and/or winding number of the insulation tape.
A common mode choke coil constructed is particularly effective for providing good characteristics for a band-pass filter to be inserted in a transmission line balanced with respect to the ground.